1. Field of the Invention
The present invention relates to a method and device for detecting a noise component included in a digital video signal, and a noise reduction method and device.
2. Description of the Related Art
A conventional noise reduction device for reducing a noise component included in a video signal is shown in FIG. 4. In FIG. 4, reference numeral 1 denotes a video signal input unit to which a digital image signal is input; 2, a noise reducing process unit for processing pixel data d and e between adjacent sequential frames to perform a noise reducing process; and 3′, a noise detecting process unit for comparing the pixel data a and b between adjacent sequential frames to perform noise detection. Reference numeral 4 denotes a selecting unit which selects and output pixel data i processed by the noise reducing process unit 2 when noise detection is performed by the noise detecting process unit 3′ and which selects and directly output pixel data h input to the video signal input unit 1 when noise detection is not performed. Reference numeral 5 denotes a 1-frame delay unit constituted by a 2-port memory or the like which delays an output image signal from the selecting unit 4 by one frame, and reference numeral 6 denotes a video signal output unit.
The noise reducing process unit 2, as shown in FIG. 5, is constituted by an adder 21 for adding pixel data d of a current image signal input to the video signal input unit 1 and pixel data e, output from the 1-frame delay unit 5, of the same pixel of in one previous frame, and a divider 22 which divides output data from the adder 21 by ½. In the divider 22, when a fractional figure (less than “1”) is obtained, the fractional figure is rounded down because the value is less than the LSB.
The noise detecting process unit 3′, as shown in FIG. 6, a subtractor 31 which calculates a difference between pixel data a of the current video signal input to the video signal input unit 1 and pixel data b, output from the 1-frame delay unit 5, of the same pixel in one previous frame, an absolute value calculator 32 which calculates an absolute value of the difference obtained by the subtractor 31, and a comparator 33 which compares a pixel data p from the absolute value calculator 32 with a preset reference value x to output a determination signal of “noise is present” when p≦x is satisfied.
An operation of the noise reduction device will be described below with reference to FIG. 7. An upper half in FIG. 7 shows pixel data (4×4 pixels for descriptive convenience) of an input image input to the video signal input unit 1, and the lower half shows similar pixel data of an output image output from the video signal output unit 6. A case in which the reference value x in the noise detecting process unit 3′ is given by x=4 on the assumption that luminance levels of the pixels are set to be levels of “0” to “255” (8 bits) will be described below.
In the noise reducing process unit 2 and the noise detecting process unit 3′, pixel data of a current input image at the same pixel and pixel data of an output image in one previous frame are to be processed. For example, an input image IN2 and an output image OUT1 in one previous frame are compared with each other. The pixel data of all the pixels of the input image IN2 are “20”, and pixel data of all the pixels are “19”. For this reason, the noise detecting process unit 3′ determines the value as “noise is present”. The noise reducing process unit 2 calculates “(19+20)/2→19” (number less than 1 is rounded down). Therefore, at this time, the output from the noise reducing process unit 2 is selected by the selecting unit 4, and pixel data “19” is output as the output image OUT2.
However, in this processing method, a problem is posed when a thing traverses a certain background, for example. A case in which an image having pixel data “80” moves from the right to the left on a background having pixel data “20” like a change of input image IN4→input image IN5 will be considered.
In an output image OUT4 processed by the input image IN4 and the output image OUT3, in a pixel where pixel data “20” of the input image IN4 is compared with the pixel data “19” of the output image OUT3, the pixel data is changed into pixel data “19”. In a pixel where pixel data “80” of the input image IN4 is compared with the pixel data “19” of the output image OUT3, the difference is larger than “4”, noise detection is not performed, and the pixel data is kept at “80”.
In an output image OUT5 processed by the input image IN5 and the output image OUT4, pixel data is changed into the pixel data “19” in a pixel where the pixel data “20” of the input image IN5 is compared with the pixel data “19” of the output image OUT4. However, since a difference between the pixel data “80” and the pixel data “19” is larger than “4” in the pixel where the pixel data “80” of the input image IN5 and the pixel data “19” of the output image OUT4 are compared with each other, noise detection is not performed, and the pixel data is kept at “80”. Since a difference between the pixel data “20” and the pixel data “80” in the pixel where the pixel data “20” of the input image IN5 is compared with the pixel data “80” of the output image OUT4, noise detection is not performed, and the pixel data is kept at “20”.
In an output image OUT6 processed by an input image IN6 and the output image OUT5, pixel data is changed into the pixel data “19” in a pixel where the pixel data “20” of the input image IN6 is compared with the pixel data “19” of the output image OUT5. However, since a difference between the pixel data “20” and the pixel data “80” is larger than “4” in a pixel where the pixel data “20” of the input image IN6 is compared with the pixel data “80” of the output image OUT5, the pixel data is kept at “20”. Furthermore, the pixel data is kept at “20” in the pixel where the pixel data “20” of the input image IN6 is compared with the pixel data “20” of the output image OUT5.
In this manner, in the output images OUT5 and OUT6, a new boundary between the pixel data “19” and the pixel data “20” is generated and disadvantageously appears as a large noise component.